Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. The large variety of active sites in Ziegler-Natta catalysts makes it difficult to control polymer architecture. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts.
Slurry reactors are in widespread use for production of polyethylene homo- and copolymers. Slurry reactors include stirred-tank reactors and water-jacketed tubular reactors arranged in a series of continuous horizontal or vertical loops. A “slurry solvent” in which polyethylene has low solubility constitutes the continuous phase in such reactors. The slurry is intensely stirred in a continuous stirred-tank reactor or series of reactors or, in the case of slurry loop reactors, is driven around the loop at relatively high speed by one or more rather massive pumps. Ethylene, supported catalyst, comonomers, and processing additives are injected into the reactor where polymerization takes place, creating a slurry of polyethylene in solvent.
U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands. Indenoindolyl catalysts are remarkably versatile because substituent effects and bridging changes can often be exploited to provide polymers with tailored physical or mechanical properties. Unbridged indenoindolyl complexes (as exemplified in the '260 patent) usually provide favorable activity although they sometimes fail to provide polymers having high enough molecular weights. Bridged indenoindolyl complexes (as taught, e.g., in U.S. Pat. No. 6,908,972) readily copolymerize α-olefins and provide polymers, with varying levels of long-chain branching. Some of the examples (e.g. Example 15 reports no long-chain branching and Mw=90,700) have very low long-chain branching (for a discussion of long-chain branching in polyethylene, see Macromolecules 39 (2006) 1474 and references cited therein).
Despite the considerable experience with single-site catalysts generally and indenoindolyl catalysts in particular, there is a need for improvement. Often, catalysts that provide good incorporation of α-olefins also produce polyethylene with too much long-chain branching, which adversely impacts polymer properties. A desirable process would easily incorporate α-olefins in order to control density and other properties. Many properties improve with increasing molecular weight, so a process capable of forming high-molecular-weight polyethylene is also desirable. Finally, it is also important to be able to adjust polymer molecular weight, preferably by adding hydrogen to the polymerization; a process with good hydrogen sensitivity is robust and versatile.
Thus, some processes incorporate α-olefins well, but have high long-chain branching; others provide polyolefins with little or no long-chain branching, but have inferior incorporation of α-olefins. Some processes incorporate α-olefins and give polymers with low long-chain branching, but are not capable of forming high-molecular-weight polyethylene or have low activity. Some processes can give high-molecular-weight polyethylene; others have good hydrogen sensitivity. However, a slurry polymerization process having all four of these qualities is apparently not known and a compromise must be made. A valuable process would enable all four attributes.